South Korea’s spy agency is investigating a major umbrella labor union for possible espionage with ties to North Korea.

This is the first raid by the National Intelligence Service on the Korean Confederation of Trade Unions, the more militant of two giant labor groups, on charges of violating the National Security Act.

According to sources within the spy agency, it is tracking whether members of the labor group received funding by North Korean agents and used the money to build an underground network in the South, influencing the union's policies and activities.

The agency suspects four members of the union were involved.

It has so far found that a member contacted personnel from the Cultural Exchange Bureau, the North Korean Labor Party's South Korean operations department, in China, Cambodia and Vietnam while others also face similar suspicions.